Regarding post #25, Wuzak.
Allison did not develop the external auxiliary supercharger because they preferred their engines to be as modular as possible.
They developed their auxiliary supercharger that way because they tried and failed at least three times to get the government to fund the development of an integral, 2-stage supercharger and were contractually bound to deliver V-1710 engines per an approved drawing set. Since they were denied funding for that development, they developed a bolt-on supercharger that worked but was less than what might have been developed had the effort been approved and funded. Allison did not have deep pockets and even General Motors was reluctant to develop a major improvement on their own money that was repeatedly denied funding by the government. They were pretty sure, and I'd have to agree with them, that any such development would be eagerly used by the government, but no payment for development would be forthcoming.
The government didn't fund the Northrop F-20 either and didn't support it when Northrop tried to market it. In the end, that VERY promising aircraft's full development effort was funded by Northrop. Considering that event, I doubt very seriously that any current US warplane maker will EVER develop a warplane on their own, without at least some overt government interest. The French firm Dassault-Breguet learned all about that one when they developed the Super Mirage 4000 on company funds and nobody bought it. They flew it for years at airshows, and everyone was impressed, but not enough to be the first customer.
As an added note to post #61, the P-38 WAS the first 400 mph fighter in service. The XP-38 went over 400 mph in 1939. The F4U Corsair was the first single-engine, 400 mph fighter in development. The XF4U-1 went over 400 mph on 1 Oct 1940 at an average ground speed of 405 miles per hour (652 km/h) from Stratford to Hartford in the U.S.A. . The Fw 190A didn't see service until March 1941. They could just hit 400 mph when they weren't having engine problems. I believe the first Spitfire to exceed 400 mph in level flight was the LF Mk. IX, which did that in 1942. The Typhoons were still having structural failures in 1941 and were not reliable 400 mph aircraft. In fact, one broke up on a speed test and killed pilot Phillip Lucan in August 1942, which resulted in adding structural plates to the tail to prevent failure at high speed. Perhaps there is one or more that I am missing?
I am only talking about aircraft that went into military service, not prototypes or other "special" aircraft. We all know the Supermarine S-6 could hit over 400 mph. but it was hardly able to attack anything. It was sometimes hard pressed to survive a landing in a smooth harbor, much less kill anything. But id DID pave the way for the Spitfire that we all know and love.